A conventional submerged filter system for purification of sewage comprises a filtration membrane module 1 shown in FIG. 1. The filtration membrane module 1 is submerged in a processing tank 2 and has a filtration unit 3. The filter unit 3 comprises an upper casing 4 and plural membrane cartridges 5. The upper casing is in the form of box, opening at the top and bottom thereof. The upper casing 4 accommodates plural membrane cartridges 5 vertically placed in parallel to each other as properly spaced from the adjoining membrane cartridges 5. The gaps between the membrane cartridges 5 form passages. The membrane cartridge 5 comprises a flat filter plate, whose surface is covered with filtration membrane. The filter plate of the membrane cartridge 5 includes a sump for collecting permeated liquid through the filtration membrane, and the sump forms a part of the passage of permeated liquid in the membrane cartridge 5.
A lower casing 6 continuous to the upper casing 4 includes a diffuser 7 therein, which spurts upward an aeration gas A including oxygen and air supplied from a source. The aeration gas spurted from the diffuser 7 generates an upward flow due to an air lift effect, making a gas-liquid cross flow along the surface of the membrane cartridges 5 adjoining each other.
An end of a suction pipe 8 communicates with a liquid collecting pipe 9a, which communicates through a flexible suction tube 9b with the passages of permeated liquid defined between the respective membrane cartridges 5. The other end of the suction pipe 8 is connected to the suction side of a sucking pump 10, while the discharge side of the sucking pump 10 is connected to a discharge pipe 11.
In processing, the sucking pump 10 is driven to apply a sucking pressure to the passages of permeated liquid between the membrane cartridges 5, thereby filtering the processed liquid. The permeated liquid through the filtration membrane is sucked by the sucking pump 10 through the suction tube 9b, liquid collecting pipe 9a and suction pipe 8, and is supplied to the following system through the discharge pipe 11.
In this process, a gas-liquid upward flow B flows as a cleaning stream along the membrane surfaces of the passages between the respective membrane cartridges 5, thereby restraining the adhesion of cake layers to the membrane surfaces.
Since the conventional membrane cartridge 5 employs a solid-core matter for the filter plate, a thin layer is formed in the passage of permeated liquid between the filtration membrane and on the surface of the filter plate, causing a flow resistance to grow. Because of the thin layer so formed, the transmembrane pressure is not applied evenly to the whole surface of the filtration membrane of the membrane cartridge 5. Accordingly, the transmembrane pressure concentrates on the vicinity of the sump communicated with the suction tube 9, so that the filtration is localized around the sump. If the filtration is localized at some part of the filtration membrane, the fouling at the part grows faster.
As shown in FIG. 2, the aeration gas spurted from a diffusing port 7a at the upper part of the diffuser 7 goes up as spreading far and wide, and flows as mixed with the processed liquid entering from the lower opening of the lower casing 6. However, if a distance between the lower end of the membrane cartridges 5 and the diffuser 7 is as short as less than 500 mm, the aeration gas will not fully spread and so the gas-liquid upward flow will be localized around the central part of the membrane cartridges 5. Further, a part of the gas-liquid upward flow collides the lower end of the membrane cartridges 5, flowing horizontally to collide the lower casing 8, thus ending up in a swirl. Due to the effect of the swirl, the flow rate of the gas-liquid cross flow between the membrane cartridges 5, becomes higher at the central part of the membrane cartridges and lower at the peripheral part thereof. Consequently, the cake layer is localized at peripheral part of the filtration membrane, and therefore the cleaning effect of the gas-liquid cross flow is not evenly utilized, causing the cake layer to grow faster.